Level Ups
On every Level Up you can choose a Reward - some of them specific to the Hero you are playing. There are special Rewards on levels 5, 10 and 20 which let you upgrade your leftmost item and a special Pedestal Encounter on levels 15 and 25 that adds an Enchantment to one of your items.
Levels 5 + 10 + 20

Upgrade
Upgrade your leftmost item.
Levels 15 + 25

Guardian's Gorge
Enchant an item with Shield effects.

Tranquil Spring
Enchant an item with Heal effects.

Celestial Conduit
Enchant an item with Cannot be Frozen, Slowed or Destroyed.

Burning Caldera
Enchant an item with Burn effects.

Murkwood Bayou
Enchant an item with Poison effect.

Languid Dunes
Enchant an item with Slow effects.

Bladeborn Badlands
Enchant an item with Crit effects.

Sanguine Valley
Enchant an weapon with Lifesteal.

Sirocco Steppe
Enchant an item with Haste effects.

Frozen Tomb
Enchant an item with Freeze effects.
Other Levels

Forage
Get 1 citrus and 1 coconut. If you have a friend, gain an additional citrus and coconut.

Meditation
Gain 2 XP

Arms Locker
Get a weapon.

Go Fishing
Get an Aquatic item.

Befriend
Get a Friend.

Load Up
Get an Ammo item.

Titan Protocol
Gain 25 Max Health per Level.

Sponsorship
Gain 1 Income per Level.

Scavenger Hunt
Get 2 Loot items. If you have a Friend, get an additional 2 items.

Shields Up
Get a Shield item.

Cash Out
Get 5 Spare Change.

Tool Up
Get a Tool.

Chocolate Connoisseur
Get 2 Chocolate Bars, then upgrade all your Chocolate Bars.

Coin Collector
Get 2 Spare Change, then upgrade all your Spare Change.

Appraisal
Your leftmost item gains 3 Value.

Pig Out
Get 5 Chocolate Bars or Gumballs.

Healthy Living
Get a Heal item.

Invest
Gain 2 Income. If you have a Property, gain an additional 2 Income.

Capital Gains
Gain 2 Income. If you have at least 25 Gold, gain an additional 2.

Opportunity
Get an Economy item.

Piggles Promo Pack
Get 2 Piggles, then upgrade all your Piggles.

Piggles Promo Pack
Get 2 Piggles, then upgrade all your Piggles.

Barrier Protocol
Your Core gains 3 Shield for each non-Weapon you have (including stash).

Barrier Protocol
Your Core gains 3 Shield for each non-Weapon you have (including stash).

Escalation Protocol
Your Core's cooldown is reduced by 5%.

Showcase Arsenal
Your leftmost weapon gains 2 damage for each Weapon you have.

Landlord
Gain 7 gold for each Property you have.

Speed Racer
Your Vehicles' cooldowns are reduced by 5%.

Handyman
Gain 50 Max Health for each Tool you have.

Personal Trainer
Spend all of your gold, gain 25 Max Health for each gold spent.

Side Hustle
Gain 2 gold for each Tool you have.

Heat Sync
Your Burn items have their cooldowns reduced by 3%.

Just Keep Swimming
Your Aquatic items have their cooldowns reduced by 3%.

Friendship Ring
Your Friends have their cooldown reduced by 3%.

Hearty Breakfast
Gain 10% Max Health.

Force Multiplier
Your weapons gain 1 damage per Level.

Force Amplifier
Your leftmost Weapon gains 2 damage per Level.

Heal Multiplier
Your Heal items gain 1 Heal per Level.

Heal Amplifier
Your leftmost Heal item gains 2 Heal per Level.

Shield Multiplier
Your Shield items gain 1 Shield per Level.

Shield Amplifier
Your leftmost Shield item gain 2 Shield per Level.

Shieldcraft
Upgrade your leftmost Shield item.

Medic's Aid
Upgrade your leftmost Heal item.

Reinforce Firewall
Your Shield items gain 4 Shield.

Friendship is Magic
Your Weapon Friends gain +3 damage.
Your Heal Friends gain 3 Heal.
Your Shield Friends gain 3 shield.

Ignite
Your Burn items gains 1 Burn.

Venomous
Your Poison items gain 1 Poison.

Hydrate
Upgrade your lefmost Aquatic item.

Locked and Loaded
Your leftmost Ammo item gains 1 Max Ammo.

Reconfigure
Your Tools have their cooldowns reduced by 5%.

Friends with Benefits
Your items gain 2% Crit Chance. If you have a friend, your items gain an additional 2% Crit Chance.

No Pain No Gain
Lose 10% Max Health. Your Weapons gain +10 damage and gain +10% Crit Chance.

Leadership
Upgrade your leftmost Weapon.

Weaponcraft
Your leftmost weapon gains 5 damage. If you have a Tool, it gains an additional 5 damage.

Optimization Protocol
Upgrade the Core.

Firestorm Protocol
Your Core gives +1 Burn to the items on the right.

Barrier Protocol
Your Core gains 3 Shield for each non-Weapon you have (including stash).

Assault Protocol
Your Core gains 3 damage for each Weapon you have (including stash).